Microlithography processes for making miniaturized electronic components, such as in the fabrication of computer chips and integrated circuits, involve using photoresists. Generally, a coating or film of a photoresist is applied to a substrate material, such as a silicon wafer used for making integrated circuits. The substrate may contain any number of layers or devices thereon.
The photoresist coated substrate is baked to evaporate any solvent in the photoresist composition and to fix the photoresist coating onto the substrate. The baked coated surface of the substrate is next subjected to selective radiation; that is, an image-wise exposure to radiation. This radiation exposure causes a chemical transformation in the exposed areas of the photoresist coated surface. After selective exposure, the photoresist coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the photoresist (depending upon whether a positive photoresist or a negative photoresist is utilized) resulting in a patterned or developed photoresist. Many developer solutions contain water and a base, such as water and a hydroxide compound.
Treating a selectively exposed photoresist with a developer conventionally involves depositing the liquid developer solution over the photoresist clad substrate and spinning the substrate whereby the liquid developer solution and dissolved areas of the photoresist are removed from the substrate by centrifugal forces. A rinsing solution, typically water, is then deposited over the photoresist clad substrate and the substrate is spun again to remove the water and any debris solubilized by the water. Spinning the substrate is a convenient and inexpensive method of removing materials from substrate. However, electrostatic charges build up on the developed photoresist during the development and water rinse cycles. Negative charges are particularly encountered on developed photoresists. While the causes of this phenomenon are not completely understood, it is believed that electrostatic charges and/or resist developer and/or deionized water rinse contribute to charge accumulation. Charge accumulation on developed photoresists can be as high as 300-400 volts, and it is typically negative.
Electrostatic charges also build up on the developed photoresist that may be used as a standard for calibrating a scanning electron microscope (SEM) or an atomic force microscope (AFM). A standard developed photoresist has at least one feature having a known size or dimension, such as a line having a linewidth of 0.20 .mu.m (additional features also of known sizes, the same or different, may also be present). Repeated use of the same standard developed photoresist for calibrating a SEM or AFM tends to induce an undesirable charge build up. In as little as 3 days of daily calibration, electrostatic charges build up on the standard developed photoresist to an extent where a feature on the standard developed photoresist appears larger than it actually is. This turn subsequently leads to inaccurate linewidth measurements due to miscalibration of a SEM or AFM.
Negative charge accumulation on a developed photoresist presents a number of problems. One notable problem is that measurement of various resist features, such as linewidth and profiling, is rendered inaccurate. Especially when using a SEM or an AFM, it is difficult to obtain accurate data. This is because SEMs and AFMs use an electron beam for generating images (both in projection and detection). The electron beam from the SEM or AFM may be repulsed by the negative charge accumulated on the photoresist. The degree of repulsion or deviation from an ideal direction is dependent upon the magnitude of the accumulated negative charge.
Another aspect of this problem is that secondary electrons emitted from the surface of the features under measurement (such as the patterned photoresist) are reduced due to negative charge accumulation. As a result, the secondary electron signal received at the detector does not represent the correct signal (as in instances where electrostatic charges do not exist on the patterned photoresist). Hence, in the case of a linewidth measurement having a developed photoresist with negative charge accumulation thereon, the SEM does not accurately measure the width because the secondary electron signal is reduced.
This phenomenon is shown in FIGS. 1 and 2. Referring to FIG. 1, SEM 10 emits an electron beam (represented by the arrow(s)) from tip 11 towards a developed photoresist structure 12 on semiconductor substrate 14. The developed photoresist structure 12 has an accumulation of negative charge 16 as a result of the lithography process and/or repeated use as a standard. Due to repulsion between the electron beam and the negative charge 16 of the developed photoresist structure 12, the electron beam path is altered away from the developed photoresist structure 12 without having reflected off or contacting the developed photoresist structure 12. Since the electron beam is not incident on the developed photoresist structure 12, an accurate measure/profile of the structure cannot be obtained. Detection of the altered electron beams by detector 18 of SEM 10 provides data indicating at least one of inaccurate linewidth, fuzzy corner definition, and otherwise non-focused images. Assessment of the quality and parameters of a lithography process is consequently difficult or inaccurate and often impossible.
Referring to FIG. 2, SEM 10 emits an electron beam (represented by the arrow(s)) from tip 11 towards a developed photoresist structure 12 on semiconductor substrate 14. The developed photoresist structure 12 has an accumulation of negative charge 16 as a result of the lithography process and/or repeated use as a standard. Due to the negative charge 16 of the developed photoresist structure 12, the secondary electron signal is reduced; that is, the accumulation of negative charges 16 interferes with the ability of the electron beam to neatly reflect off of and be received by detector 18 of SEM 10.
This is especially true with regard to measuring linewidth and with using photoresists sensitive to small wavelengths of light. Photoresists sensitive to relatively small wavelengths of light are typically formed or developed into finer patterns (e.g., smaller linewidths) compared to photoresists sensitive to relatively large wavelengths of light. Linewidth measurements taken on undesirably charged photoresists often yield parameters in error, sometimes by an order of magnitude. For example, an SEM scanning an undesirably charged photoresist having a linewidth of 0.20 .mu.m may incorrectly indicate that the linewidth is 0.24 .mu.m. This is primarily due to charge induced deviation of the electron beam used to measure the linewidth. Such errors cannot be tolerated because they lead to fatal construction errors in subsequent processing.
Referring to FIG. 3, the critical dimension data graph of a secondary electron profile of the developed photoresist of FIG. 1 using an SEM is shown by the solid line. An accurate measurement of the developed photoresist of FIG. 1 should yield the dashed line of FIG. 3, but due to undesirable charge accumulation, the apparent critical dimension is inaccurately wide. Accurate critical dimension measurements are desired.